This invention pertains to differential type torque limiting devices for mechanical power transmission, and more particularly, to such devices having a single input with two outputs driving two independent actuators which in turn drive a single control surface or device which will tolerate only limited asymmetric loads.
Typically, a single moveable auxiliary airfoil, e.g. a horizontal stabilizer may be actuated by a single actuator although it may employ a torque tube and multiple linkages. However, a single actuator restricts provisions for suitable redundancy in the system in the event of failure of the actuator, its structural support, or its connection to the control surface. Particularly acute is a horizontal stabilizer trim system wherein the entire stabilizer is rotated to provide trim. In the event of an actuator load path failure, the entire stabilizer would be free to rotate about its hinge line and the elevators could not compensate. Dual actuators, either of which will support the stabilizer, but not necessarily drive it alone, provide adequate redundancy. Acme jackscrews provide these requirements. However, employment of dual actuators necessitates not only synchronization of the actuators but the airfoil section rotated must be structurally capable of withstanding a failed or jammed single actuator while the remaining actuator applies the force associated with the stall torque of the drive means. Alternately, provisions must be made to avoid this load condition.
This inventive device was employed to modify an actual system in commercial use which employed a redundant chain drive to each actuator to avoid the failed chain load condition. Additionally, a chain failure monitoring system was incorporated as a single failed chain could not be detected short of visual inspection. Obviously, without the detection system, a single chain could fail to either or both actuators, the system would function as usual, but the benefit of the redundancy would be lost. The benefit of the redundancy was also lost if the detection system experienced a failure.
The innovative device of this invention was employed in the system which eliminated the redundant chains and sprockets as well as the associated idlers, chain protectors required to prevent a failed chain from jamming an operative chain and the failure monitor. The net result was a fused torque detection drive system which prevents structural overloads of the control surface in case of a failed chain or a jammed actuator as well as realizing an 80 pound weight reduction and enhanced system reliability. Of course, in either case, the surface being controlled must be of a nature that its loss will not totally debilitate the vehicle as in the case of a trim tab where the prime surface is independently actuated.